U.S. patent number 9,513,480 [Application Number 14/617,697] was granted by the patent office on 2016-12-06 for waveguide.
This patent grant is currently assigned to Microsoft Technology Licensing, LLC. The grantee listed for this patent is Microsoft Technology Licensing, LLC. Invention is credited to Pasi Kostamo, Pasi Saarikko.
United States Patent |
9,513,480 |
Saarikko , et al. |
December 6, 2016 |
Waveguide
Abstract
A waveguide has a front and a rear surface, the waveguide for a
display system and arranged to guide light from a light engine onto
an eye of a user to make an image visible to the user, the light
guided through the waveguide by reflection at the front and rear
surfaces. A first portion of the front or rear surface has a
structure which causes light to change phase upon reflection from
the first portion by a first amount. A second portion of the same
surface has a different structure which causes light to change
phase upon reflection from the second portion by a second amount
different from the first amount. The first portion is offset from
the second portion by a distance which substantially matches the
difference between the second amount and the first amount.
Inventors: |
Saarikko; Pasi (Espoo,
FI), Kostamo; Pasi (Espoo, FI) |
Applicant: |
Name |
City |
State |
Country |
Type |
Microsoft Technology Licensing, LLC |
Redmond |
WA |
US |
|
|
Assignee: |
Microsoft Technology Licensing,
LLC (Redmond, WA)
|
Family
ID: |
55398452 |
Appl.
No.: |
14/617,697 |
Filed: |
February 9, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20160231568 A1 |
Aug 11, 2016 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
5/1861 (20130101); G02B 6/0018 (20130101); G02B
6/0016 (20130101); G02B 27/0172 (20130101); G02B
6/0033 (20130101); G02B 27/4272 (20130101); G02B
5/1842 (20130101); G02B 5/1866 (20130101); G02B
27/0101 (20130101); G02B 6/0035 (20130101); G02B
2027/0123 (20130101); G02B 2027/011 (20130101); G02B
2027/0178 (20130101) |
Current International
Class: |
G02B
6/26 (20060101); F21V 8/00 (20060101); G02B
5/18 (20060101); G02B 27/01 (20060101) |
Field of
Search: |
;385/31,37 ;349/65
;359/630,34,571,633 ;345/690 |
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|
Primary Examiner: Kim; Ellen
Attorney, Agent or Firm: Goldsmith; Micah Yee; Judy Minhas;
Micky
Claims
The invention claimed is:
1. A waveguide having a front and a rear surface, the waveguide for
a display system and arranged to guide light from a light engine
onto an eye of a user to make an image visible to the user, the
light guided through the waveguide by reflection at the front and
rear surfaces; wherein a first portion of the front or rear surface
has a structure which causes light to change phase upon reflection
from the first portion by a first amount; wherein a second portion
of the same surface has a different structure which causes light to
change phase upon reflection from the second portion by a second
amount different from the first amount; and wherein the first
portion is offset from the second portion by a distance which
substantially matches the difference between the second amount and
the first amount.
2. A waveguide according to claim 1 wherein the structure of the
first portion constitutes a first diffraction grating.
3. A waveguide according to claim 2 wherein the structure of the
second portion constitutes a second diffraction grating.
4. A waveguide according to claim 3 wherein the first grating has a
depth different from the second grating.
5. A waveguide according to claim 2 wherein the first grating has a
depth which is substantially constant over the entire first portion
up to the edge of the first grating.
6. A waveguide according to claim 3 wherein the first grating has a
depth which is substantially constant over the entire first portion
up to the edge of the first grating, and the second grating has a
depth which is substantially constant over the entire second
portion up to the edge of the second grating.
7. A waveguide according to claim 1 wherein the structure of the
first portion constitutes a first diffraction grating and the
structure of the second portion is substantially
non-diffractive.
8. A waveguide according to claim 7 wherein the first grating has a
depth which is substantially constant over the entire first portion
up to the edge of the first grating.
9. A waveguide according to claim 1 wherein the first and second
portions are substantially contiguous.
10. A waveguide according to claim 9, wherein the first and second
portions are separated by no more than 100 micrometers in width
along a common border, and optionally no more than 50 micrometers
in width along the common border.
11. A waveguide according to claim 1 wherein a third portion of the
same surface has a structure which causes light to change phase
upon reflection from the third portion by a third amount different
from the first amount, wherein the first and third portions are
adjacent the second portion so that the second portion separates
the first and third portions, and wherein the third portion is
offset from the second portion by a distance which substantially
matches the difference between the second amount and the third
amount.
12. A waveguide according to claim 11 wherein the structure of the
first portion constitutes a first diffraction grating, the
structure of the third portion constitutes a second diffraction
grating, and the structure of the second portion is substantially
non-diffractive.
13. An image display system comprising: a light engine configured
to generate an image; a waveguide having a front and a rear
surface, the waveguide arranged to guide light from the light
engine onto an eye of a user to make the image visible to the user,
the light guided through the waveguide by reflection at the front
and rear surfaces, wherein a first portion of the front or rear
surface has a structure which causes light to change phase upon
reflection from the first portion by a first amount, wherein a
second portion of the same surface has a different structure which
causes light to change phase upon reflection from the second
portion by a second amount different from the first amount, and
wherein the first portion is offset from the second portion by a
distance which substantially matches the difference between the
second amount and the first amount.
14. A display system according to claim 13 wherein the structure of
the first portion constitutes an incoupling grating via which said
light is coupled into the waveguide from the display.
15. A display system according to claim 13 wherein the structure of
the second portion constitutes an exit grating via which said light
exits the waveguide onto the eye of the user.
16. A display system according to claim 13 wherein the structure of
the second portion constitutes an intermediate grating configured
to manipulate the spatial distribution of the light within the
waveguide.
17. A wearable image display system comprising: a headpiece; a
light engine mounted on the headpiece and configured to generate an
image; a waveguide located forward of an eye of a wearer in use,
the waveguide having a front and a rear surface, the waveguide
arranged to guide light from the light engine onto the eye of the
wearer to make the image visible to the wearer, the light guided
through the waveguide by reflection at the front and rear surfaces,
wherein a first portion of the front or rear surface has a
structure which causes light to change phase upon reflection from
the first portion by a first amount, wherein a second portion of
the same surface has a different structure which causes light to
change phase upon reflection from the second portion by a second
amount different from the first amount, and wherein the first
portion is offset from the second portion by a distance which
substantially matches the difference between the second amount and
the first amount.
18. A display system according to claim 17 wherein the structure of
the first portion constitutes an incoupling grating via which said
light is coupled into the waveguide from the display.
19. A display system according to claim 17 wherein the structure of
the second portion constitutes an exit grating via which said light
exits the waveguide onto the eye of the user.
20. A display system according to claim 17 wherein the structure of
the second portion constitutes an intermediate grating configured
to manipulate the spatial distribution of the light within the
waveguide.
Description
BACKGROUND
Display systems can used to make a desired image visible to a user
(viewer). Wearable display systems can be embodied in a wearable
headset which is arranged to display an image within a short
distance from a human eye. Such wearable headsets are sometimes
referred to as head mounted displays, and are provided with a frame
which has a central portion fitting over a user's (wearer's) nose
bridge and left and right support extensions which fit over a
user's ears. Optical components are arranged in the frame so as to
display an image within a few centimeters of the user's eyes. The
image can be a computer generated image on a display, such as a
micro display. The optical components are arranged to transport
light of the desired image which is generated on the display to the
user's eye to make the image visible to the user. The display on
which the image is generated can form part of a light engine, such
that the image itself generates collimated lights beams which can
be guided by the optical component to provide an image visible to
the user.
Different kinds of optical components have been used to convey the
image from the display to the human eye. These can include lenses,
mirrors, optical waveguides, holograms and diffraction gratings,
for example. In some display systems, the optical components are
fabricated using optics that allows the user to see the image but
not to see through this optics at the "real world". Other types of
display systems provide view through this optics so that the
generated image which is displayed to the user is overlaid onto a
real world view. This is sometimes referred to as augmented
reality.
Waveguide-based display systems typically transport light from a
light engine to the eye via a TIR (Total Internal Reflection)
mechanism in a waveguide (light guide). Such systems can
incorporate diffraction gratings, which cause effective beam
expansion so as to output expanded versions of the beams provided
by the light engine. This means the image is visible over a wider
area when looking at the waveguide's output than when looking at
the light engine directly: provided the eye is within an area such
that it can receive some light from substantially all of the
expanded beams, the whole image will be visible to the user. Such
an area is referred to as an eye box.
To maintain image quality, the structure of the waveguide can be
configured in various ways to mitigate distortion of the
transported light.
SUMMARY
This summary is provided to introduce a selection of concepts in a
simplified form that are further described below in the detailed
description. This summary is not intended to identify key features
or essential features of the claimed subject matter, nor is it
intended to be used to limit the scope of the claimed subject
matter. Nor is the claimed subject matter limited to
implementations that solve any or all of the disadvantages noted in
the background section.
According to a first aspect a waveguide has a front and a rear
surface. The waveguide is for a display system and is arranged to
guide light from a light engine onto an eye of a user to make an
image visible to the user. The light is guided through the
waveguide by reflection at the front and rear surfaces. A first
portion of the front or rear surface has a structure which causes
light to change phase upon reflection from the first portion by a
first amount. A second portion of the same surface has a different
structure which causes light to change phase upon reflection from
the second portion by a second amount different from the first
amount. The first portion is offset from the second portion by a
distance which substantially matches the difference between the
second amount and the first amount.
According to a second aspect an image display system comprises a
light engine configured to generate an image and a waveguide having
a front and a rear surface. The waveguide is arranged to guide
light from the light engine onto an eye of a user to make the image
visible to the user, the light guided through the waveguide by
reflection at the front and rear surfaces. A first portion of the
front or rear surface has a structure which causes light to change
phase upon reflection from the first portion by a first amount. A
second portion of the same surface has a different structure which
causes light to change phase upon reflection from the second
portion by a second amount different from the first amount. The
first portion is offset from the second portion by a distance which
substantially matches the difference between the second amount and
the first amount.
According to a third aspect a wearable image display system
comprising: a headpiece; a light engine mounted on the headpiece
and configured to generate an image; and a waveguide located
forward of an eye of a wearer in use. The waveguide has a front and
a rear surface, and is arranged to guide light from the display
onto the eye of the wearer to make the image visible to the wearer,
the light guided through the waveguide by reflection at the front
and rear surfaces. A first portion of the front or rear surface has
a structure which causes light to change phase upon reflection from
the first portion by a first amount. A second portion of the same
surface has a different structure which causes light to change
phase upon reflection from the second portion by a second amount
different from the first amount. The first portion is offset from
the second portion by a distance which substantially matches the
difference between the second amount and the first amount.
BRIEF DESCRIPTION OF FIGURES
FIG. 1 shows a wearable display system;
FIG. 2A shows a plan view of part of the display system;
FIGS. 3A and 3B shows perspective and frontal view of an optical
component;
FIG. 4A shows a schematic plan view of an optical component having
a surface relief grating formed on its surface;
FIG. 4B shows a schematic illustration of the optical component of
FIG. 4A, shown interacting with incident light and viewed from the
side;
FIG. 5A shows a schematic illustration of a straight binary surface
relief grating, shown interacting with incident light and viewed
from the side;
FIG. 5B shows a schematic illustration of a slanted binary surface
relief grating, shown interacting with incident light and viewed
from the side;
FIG. 5C shows a schematic illustration of an overhanging triangular
surface relief grating, shown interacting with incident light and
viewed from the side;
FIG. 6 shows a close up view of part of an incoupling zone of an
optical component;
FIG. 7A shows a perspective view of a part of a display system;
FIG. 7B shows a plan view of individual pixels of a display;
FIGS. 7C and 7D show plan and frontal views of a beam interacting
with an optical component;
FIG. 7E shows a frontal view of an optical component performing
beam expansion;
FIG. 7F shows a plan view of an optical component performing beam
expansion;
FIG. 7G is a plan view of a curved optical component;
FIGS. 8A and 8B are plan and frontal views of a part of an optical
component;
FIG. 9A shows a perspective view of beam reflection within a fold
zone of a waveguide;
FIG. 9B illustrates a beam expansion mechanism;
FIGS. 10A and 10B show plan and side views of a part of a waveguide
having optical elements which are not offset from one another, and
FIG. 10C shows a phase distribution for the waveguide of FIGS. 10A
and 10B;
FIG. 11A shows a side view of a part of a waveguide having optical
elements which are not offset from one another and which exhibit
apodization, and FIG. 11B shows a phase distribution for the
waveguide of FIG. 11A.
FIG. 12A shows a side view of a part of a first optical component
having offset optical elements, and FIG. 12B shows a phase
distribution for the second waveguide of FIG. 12A;
FIGS. 13A and 13B show plan and side views of a part of a second
waveguide having offset optical elements, and FIG. 13C shows a
phase distribution for the second waveguide of FIGS. 13A and
13B;
FIG. 14 shows graphs of simulated performance data for the
waveguide of FIGS. 11A and 11B;
FIG. 15 shows graphs of simulated performance data for the first
waveguide of FIG. 12A;
FIG. 16 shows a flow chart for a microfabrication process for
manufacturing optical components or masters;
FIG. 17A shows an exemplary optical component having certain
characteristics which may impact on image quality;
FIG. 17B shows an exposure set-up which could be used in making the
optical component of FIG. 17A;
FIG. 18 shows a graph of MTF as function of gap width for an
exemplary waveguide.
DETAILED DESCRIPTION
Typically, a waveguide based display system comprises an image
source, e.g. a projector, waveguide(s) and various optical elements
(e.g. diffraction gratings) imprinted on the waveguide
surfaces.
FIGS. 10A and 10B show side and plan views of part of an optical
waveguide 10a having diffractive optical elements O1, O2 (which are
diffraction gratings) imprinted on the top of the waveguide's
surface. The first grating O1 has a depth h1 and the second grating
O2 has a depth h2. An expanded side view of the optical elements
O1, O2 is shown at the top of FIG. 10A. Each optical element is
formed of a series of grooves in the surface of the waveguide 10a
of depth h1, h2.noteq.h1 respectively as measured normal to the
waveguide. The depths h1, h2 are constant across the whole of the
gratings O1, O2 in this example.
The first and second elements O1, O2 are used, for example, to
couple light emitted by the image source into and out of the
waveguide, and/or for manipulation of its spatial distribution
within the waveguide. While being necessary for the operation of
the display system, the optical elements O1, O2 can also cause
unwanted distortions on the phase front of the light field as it
travels through the waveguide. In particular, phase distortions may
be created when the wavefront meets the edges of the optical
elements O1, O2. Elements may also change the amplitude of the
field differently, i.e. there will be amplitude variation as well.
However, in terms of image quality the phase distortion is much
more severe and matching of amplitude of the field portions is not
required to achieve acceptable image quality.
The optical elements O1 and O2 are separated by a blank surface
region B, which is substantially non-diffractive (i.e. which
interacts with light substantially in accordance with Snell's law
and the law of reflection). Portions of the wavefront that are
totally internally reflected from the blank surface region B of the
light guide experience a different phase retardation than portions
that are reflected from the optical elements O1, O2. A ray R0
change phase upon total internal reflection from the (or any other)
blank surface region B by an amount .phi.0 which depends on the
polarization of the incident light. A ray R1 change phase upon
reflection from the first optical element O1 by an amount
.phi.1=.phi.0-.DELTA..phi.1. A ray R2 change phase upon reflection
from the second optical component O2 by an amount
.phi.2=.phi.0-.phi.2. This is illustrated in the phase distribution
of FIG. 10C, which shows the phase of the rays R1, R0, R2 after
reflection from the first optical element O1, the blank surface B,
and the second optical element O2 respectively.
Generally, gratings and TIR change the phase of polarization
components differently, i.e. there is polarization rotation as
well. As will be apparent, the description of the preceding
paragraph is a simplification to aid illustration of the distortion
mechanism.
Note the term "reflected" as it is used herein includes
reflectively diffracted light e.g. as created by a reflective or
partially reflective diffraction grating. Both zero and higher
order modes can experience phase retardation. In general,
polarization of reflected higher order modes as well as 0th order
mode can be rotated or turned into/out of elliptical polarization
etc.
Such phase jumps result in diffractive beam spreading and thus loss
of image sharpness. One method to reduce the effect of edge
diffraction would be to use apodization. Generally this means using
some form of smoothing to turn sharp edges into more continuously
varying shapes. The smoothing can be done through various means. In
the case of gratings the depth of the grating structure, or more
generally any other profile parameter, could be varied smoothly
between two regions. An exemplary waveguide 10b exhibiting
apodization is shown in FIG. 11A. The waveguide 10b has first and
second optical elements O1', O2' which are equivalent to the
optical component O1, O2 of the waveguide 10a but for the fact that
the depth of O1', O2' gradually decreases to zero throughout first
and second apodization regions A1, A2 respectively, which are
adjacent a blank region B that separates the optical elements O1'
and O2'. This results in gradually varying phase in the apodized
area, as shown in the phase distribution of FIG. 11B. As can be
seen in FIG. 11B, the amount by which the phase of rays changes due
to reflection in the apodization regions A1, A2 varies as a
function of location across the apodization regions A1, A2, with
rays reflected closer to the blank region B exhibiting phase
changes which are close to that exhibited by rays reflected in the
blank region B itself. Whilst this can in some cases lead to
reduced strength edge diffraction and diffractive beam spreading,
apodization can have other undesired effects e.g. it can reduce the
efficiency of the grating near its edges.
The present disclosure provides a means to reduce phase distortions
caused by diffractive optical elements imprinted on the surface of
the light guide. In particular, the effect of the grating edge on
the wavefront is removed by adding a suitable height offset to a
grating and/or to the blank surface (or other grating) next to it.
The offset is selected so that the total phase retardance for rays
that are reflected from the offset grating is equal to the phase
retardance of rays that are totally internally reflected from the
blank surface of the waveguide (or that are reflected from the
other grating).
As compared with the method of using apodization, the method of the
present disclosure allows for improved reduction of phase
distortions as compared with apodization. This is achieved while
maintaining other desired properties of the gratings, e.g. gratings
can be optimized for efficiency over the entire surface area of the
gratings, including at the edges of the grating, by for instance
maintaining a desired depth profile right up to the edges of the
grating.
This is described in detail below. First, a context in which the
waveguides of the present disclosure can be used will be
described.
FIG. 1 is a perspective view of a head mounted display. The head
mounted display comprises a headpiece, which comprises a frame 2
having a central portion 4 intended to fit over the nose bridge of
a wearer, and a left and right supporting extension 6,8 which are
intended to fit over a user's ears. Although the supporting
extensions are shown to be substantially straight, they could
terminate with curved parts to more comfortably fit over the ears
in the manner of conventional spectacles.
The frame 2 supports left and right optical components, labelled
10L and 10R, which are waveguides. For ease of reference herein an
optical component 10 (which is a waveguide) will be considered to
be either a left or right component, because the components are
essentially identical apart from being mirror images of each other.
Therefore, all description pertaining to the left-hand component
also pertains to the right-hand component. The optical components
will be described in more detail later with reference to FIG. 3.
The central portion 4 houses a light engine which is not shown in
FIG. 1 but which is shown in FIG. 2.
FIG. 2 shows a plan view of a section of the top part of the frame
of FIG. 1. Thus, FIG. 2 shows the light engine 13 which comprises a
micro display 15 and imaging optics 17 in the form of a collimating
lens 20. The light engine also includes a processor which is
capable of generating an image for the micro display. The micro
display can be any type of light of image source, such as liquid
crystal on silicon (LCOS) displays (LCD's), transmissive liquid
crystal displays (LCD), matrix arrays of LED's (whether organic or
inorganic) and any other suitable display. The display is driven by
circuitry which is not visible in FIG. 2 which activates individual
pixels of the display to generate an image. The substantially
collimated light, from each pixel, falls on an exit pupil 22 of the
light engine 13. At exit pupil 22, collimated light beams are
coupled into each optical component, 10L, 10R into a respective
in-coupling zone 12L, 12R provided on each component. These
in-coupling zones are clearly shown in FIG. 1, but are not readily
visible in FIG. 2. In-coupled light is then guided, through a
mechanism that involves diffraction and TIR, laterally of the
optical component in a respective intermediate (fold) zone 14L,
14R, and also downward into a respective exit zone 16L, 16R where
it exits the component 10 towards the users' eye. The zones 14L,
14R, 16L and 16R are shown in FIG. 1. These mechanisms are
described in detail below. FIG. 2 shows a user's eye (right or
left) receiving the diffracted light from an exit zone (16L or
16R). The output beam OB to a user's eye is parallel with the
incident beam IB. See, for example, the beam marked IB in FIG. 2
and two of the parallel output beams marked OB in FIG. 2. The
optical component 10 is located between the light engine 13 and the
eye i.e. the display system configuration is of so-called
transmissive type.
Other headpieces are also within the scope of the subject matter.
For instance, the display optics can equally be attached to the
users head using a head band, helmet or other fit system. The
purpose of the fit system is to support the display and provide
stability to the display and other head borne systems such as
tracking systems and cameras. The fit system will also be designed
to meet user population in anthropometric range and head morphology
and provide comfortable support of the display system.
Beams from the same display 15 may be coupled into both components
10L, 10R so that an image is perceived by both eyes from a single
display, or separate displays may be used to generate different
images for each eye e.g. to provide a stereoscopic image. In
alternative headsets, light engine(s) may be mounted at one or both
of left and right portions of the frame--with the arrangement of
the incoupling, fold and exit zones 12, 14, 16 flipped
accordingly.
The optical component 10 is substantially transparent such that a
user can not only view the image from the light engine 13, but also
can view a real world view through the optical components.
The optical component 10 has a refractive index n which is such
that total internal reflection takes place guiding the beam from
the incoupling zone along the intermediate expansion zone 14, and
down towards the exit zone 16.
FIGS. 3A and 3B show an optical component in more detail.
FIG. 3A shows a perspective view of an optical component 10. The
optical component is flat in that the front and rear portions of
its surface are substantially flat (front and rear defined from the
viewpoint of the wearer, as indicated by the location of the eye in
FIG. 3A). The front and rear portions of the surface are parallel
to one another. The optical component 10 lies substantially in a
plane (xy-plane), with the z axis (referred to as the "normal")
directed towards the viewer from the optical component 10. The
incoupling, fold and exit zones 12, 14 and 16 are shown, each
defined by respective surface modulations 52, 46 and 56 on the
surface of the optical component, which are on the rear of the
waveguide from a viewpoint of the wearer. Each of the surface
modulations 52, 46, 56 forms a respective surface relief grating
(SRG), the nature of which will be described shortly. Instead of
the SRGs, holograms could be used providing the same optical
function as the SRGs.
As shown in the plan view of FIG. 3B, the fold zone has a
horizontal extent W2 (referred to herein as the "length" of the
expansion zone) in the lateral (x) direction and an extent H2 in
the y direction (referred to herein as the "length" of the
expansion zone) which increases from the inner edge of the optical
component to its outer edge in the lateral direction along its
width W2. The exit zone has a horizontal extent W3 (length of the
exit zone) and y-direction extent H3 (width of the exit zone) which
define the size of the eye box, which size is independent of the
imaging optics in the light engine. The incoupling and fold SRGs
52, 54 have a relative orientation angle A, as do the fold and exit
SRGs 54, 56 (note the various dotted lines superimposed on the SRGs
52, 54, 56 denote directions perpendicular to the grating lines of
those SRGs).
The incoupling and fold zones 12, 14 are substantially contiguous
in that they are separated by at most a narrow border zone 18 which
has a width W as measured along (that is, perpendicular to) a
common border 19 that divides the border zone 18. The common border
19 is arcuate (substantially semi-circular in this example), the
incoupling and fold regions 12, 14 having edges which are arcuate
(substantially semi-circular) along the common border 19. The edge
of incoupling region 12 is substantially circular overall.
Principles of the diffraction mechanisms which underlie operation
of the head mounted display described herein will now be described
with reference to FIGS. 4A and 4B.
The optical components described herein interact with light by way
of reflection, refractions and diffraction. Diffraction occurs when
a propagating wave interacts with a structure, such as an obstacle
or slit. Diffraction can be described as the interference of waves
and is most pronounced when that structure is comparable in size to
the wavelength of the wave. Optical diffraction of visible light is
due to the wave nature of light and can be described as the
interference of light waves. Visible light has wavelengths between
approximately 390 and 700 nanometers (nm) and diffraction of
visible light is most pronounced when propagating light encounters
structures of a similar scale e.g. of order 100 or 1000 nm in
scale.
One example of a diffractive structure is a periodic (substantially
repeating) diffractive structure. Herein, a "diffraction grating"
means any (part of) an optical component which has a periodic
diffractive structure. Periodic structures can cause diffraction of
light, which is typically most pronounced when the periodic
structure has a spatial period of similar size to the wavelength of
the light. Types of periodic structures include, for instance,
surface modulations on the surface of an optical component,
refractive index modulations, holograms etc. When propagating light
encounters the periodic structure, diffraction causes the light to
be split into multiple beams in different directions. These
directions depend on the wavelength of the light thus diffractions
gratings cause dispersion of polychromatic (e.g. white) light,
whereby the polychromatic light is split into different coloured
beams travelling in different directions.
When the periodic structure is on the surface of an optical
component, it is referred to a surface grating. When the periodic
structure is due to modulation of the surface itself, it is
referred to as a surface relief grating (SRG). An example of a SRG
is uniform straight grooves in a surface of an optical component
that are separated by uniform straight groove spacing regions.
Groove spacing regions are referred to herein as "lines", "grating
lines" and "filling regions". The nature of the diffraction by a
SRG depends both on the wavelength of light incident on the grating
and various optical characteristics of the SRG, such as line
spacing, groove depth and groove slant angle. An SRG can be
fabricated by way of a suitable microfabrication process, which may
involve etching of and/or deposition on a substrate to fabricate a
desired periodic microstructure on the substrate to form an optical
component, which may then be used as a production master such as a
mould for manufacturing further optical components.
An SRG is an example of a Diffractive Optical Element (DOE). When a
DOE is present on a surface (e.g. when the DOE is an SRG), the
portion of that surface spanned by that DOE is referred to as a DOE
area.
FIGS. 4A and 4B show from the top and the side respectively part of
a substantially transparent optical component 10 having an outer
surface S. At least a portion of the surface S exhibits surface
modulations that constitute a SRG 44 (e.g. 52, 54, 56), which is a
microstructure. Such a portion is referred to as a "grating area".
The modulations comprise grating lines which are substantially
parallel and elongate (substantially longer than they are wide),
and also substantially straight in this example (though they need
not be straight in general).
FIG. 4B shows the optical component 10, and in particular the SRG
44, interacting with an incoming illuminating light beam I that is
inwardly incident on the SRG 44. The light I is white light in this
example, and thus has multiple colour components. The light I
interacts with the SRG 44 which splits the light into several beams
directed inwardly into the optical component 10. Some of the light
I may also be reflected back from the surface S as a reflected beam
R0. A zero-order mode inward beam T0 and any reflection R0 are
created in accordance with the normal principles of diffraction as
well as other non-zero-order (.+-.n-order) modes (which can be
explained as wave interference). FIG. 4B shows first-order inward
beams T1, T-1; it will be appreciated that higher-order beams may
or may not also be created depending on the configuration of the
optical component 10. Because the nature of the diffraction is
dependent on wavelength, for higher-order modes, different colour
components (i.e. wavelength components) of the incident light I
are, when present, split into beams of different colours at
different angles of propagation relative to one another as
illustrated in FIG. 4B.
FIGS. 5A-5C are close-up schematic cross sectional views of
different exemplary SRGs 44a-44c (collectively referenced as 44
herein) that may be formed by modulation of the surface S of the
optical component 10 (which is viewed from the side in these
figures). Light beams are denoted as arrows whose thicknesses
denote approximate relative intensity (with higher intensity beams
shown as thicker arrows).
FIG. 5A shows an example of a straight binary SRG 44a. The straight
binary SRG 44a is formed of a series of grooves 7a in the surface S
separated by protruding groove spacing regions 9a which are also
referred to herein as "filling regions", "grating lines" or simply
"lines". The SRG 44a has a spatial period of d (referred to as the
"grating period"), which is the distance over which the
modulations' shape repeats and which is thus the distance between
adjacent lines/grooves. The grooves 7a have a depth h and have
substantially straight walls and substantially flat bases. The
filling regions have a height h and a width that is substantially
uniform over the height h of the filling regions, labelled "w" in
FIG. 2A (with w being some fraction f of the period: w=f*d).
For a straight binary SRG, the walls are substantially
perpendicular to the surface S. The SRG 44a causes symmetric
diffraction of incident light I that is entering perpendicularly to
the surface, in that each +n-order mode beam (e.g. T1) created by
the SRG 4a has substantially the same intensity as the
corresponding -n-order mode beam (e.g. T-1), typically less than
about one fifth (0.2) of the intensity of the incident beam I.
FIG. 5B shows an example of a slanted binary SRG 44b. The slanted
binary SRG 44b is also formed of grooves, labelled 7b, in the
surface S having substantially straight walls and substantially
flat bases separated by lines 9b of width w. However, in contrast
to the straight SRG 44a, the walls are slanted by an amount
relative to the normal, denoted by the angle .beta. in FIG. 25B.
The grooves 7b have a depth h as measured along the normal. Due to
the asymmetry introduced by the non-zero slant, .+-.n-order mode
inward beams travelling away from the slant direction have greater
intensity that their .-+.n-order mode counterparts (e.g. in the
example of FIG. 2B, the T1 beam is directed away from the direction
of slant and has usually greater intensity than the T-1 beam,
though this depends on e.g. the grating period d); by increasing
the slant by a sufficient amount, those .-+.n counterparts can be
substantially eliminated (i.e. to have substantially zero
intensity). The intensity of the T0 beam is typically also very
much reduced by a slanted binary SRG such that, in the example of
FIG. 5B, the first-order beam T1 typically has an intensity of at
most about four fifths (0.8) the intensity of the incident beam I
but this is highly dependent on wavelength and incident angle.
The binary SRGs 44a and 44b can be viewed as spatial waveforms
embedded in the surface S that have a substantially square wave
shape (with period d). In the case of the SRG 44b, the shape is a
skewed square wave shape skewed by .beta..
FIG. 5C shows an example of an overhanging triangular SRG 44c which
is a special case of an overhanging trapezoidal SRG. The triangular
SRG 44c is formed of grooves 7c in the surface S that are
triangular in shape (and which thus have discernible tips) and
which have a depth h as measured along the normal. Filling regions
9c take the form of triangular, tooth-like protrusions (teeth),
having medians that make an angle .beta. with the normal (.beta.
being the slant angle of the SRG 44c). The teeth have tips that are
separated by d (which is the grating period of the SRG 44c), a
width that is w at the base of the teeth and which narrows to
substantially zero at the tips of the teeth. For the SRG of FIG.
44c, w.apprxeq.d, but generally can be w<d. The SRG is
overhanging in that the tips of the teeth extend over the tips of
the grooves. It is possible to construct overhanging triangular
SRGs that substantially eliminate both the transmission-mode T0
beam and the .-+.n-mode beams, leaving only .+-.n-order mode beams
(e.g. only T1). The grooves have walls which are at an angle
.gamma. to the median (wall angle).
The SRG 44c can be viewed as a spatial waveform embedded in S that
has a substantially triangular wave shape, which is skewed by
.beta..
Other SRGs are also possible, for example other types of
trapezoidal SRGs (which may not narrow in width all the way to
zero), sinusoidal SRGs etc. Such other SRGs also exhibit depth h,
linewidth w, slant angle .beta. and wall angles .gamma. which can
be defined in a similar manner to FIG. 5A-C.
In the present display system, d is typically between about 250 and
500 nm, and h between about 30 and 400 nm. The slant angle .beta.
is typically between about 0 and 45 degrees (such that slant
direction is typically elevated above the surface S by an amount
between about 45 and 90 degrees).
An SRG has a diffraction efficiency defined in terms of the
intensity of desired diffracted beam(s) (e.g. T1) relative to the
intensity of the illuminating beam I, and can be expressed as a
ratio .eta. of those intensities. As will be apparent from the
above, slanted binary SRGs can achieve higher efficiency (e.g.
4b--up to .eta..apprxeq.0.8 if T1 is the desired beam) than
non-slanted SRGs (e.g. 44a--only up to about .eta..apprxeq.0.2 if
T1 is the desired beam). With overhanging triangular SRGs, it is
possible to achieve near-optimal efficiencies of
.eta..apprxeq.1.
FIG. 6 shows the incoupling SRG 52 with greater clarity, including
an expanded version showing how the light beam interacts with it.
FIG. 6 shows a plan view of the optical component 10. The light
engine 13 provides beams of collimated light, one of which is shown
(corresponding to a display pixel). That beam falls on the
incoupling SRG 52 and thus causes total internal reflection of the
beam in the component 10. The intermediate grating 14 directs
versions of the beams down to the exit grating 16, which causes
diffraction of the image onto the user's eye. The operation of the
grating 12 is shown in more detail in the expanded portion which
shows rays of the incoming light beam coming in from the left and
denoted I and those rays being diffracted so as to undergo TIR in
the optical component 10. The grating in FIG. 6 is of the type
shown in FIG. 5B but could also be of the type shown in FIG. 5C or
some other slanted grating shape.
Optical principles underlying certain embodiments will now be
described with reference to FIGS. 7A-9B.
Collimating optics of the display system is arranged to
substantially collimate an image on a display of the display system
into multiple input beams. Each beam is formed by collimating light
from a respective image point, that beam directed to the incoupling
grating in a unique inward direction which depends on the location
of that point in the image. The multiple input beams thus form a
virtual version of the image. The intermediate and exit grating
have widths substantially larger than the beams' diameters. The
incoupling grating is arranged to couple each beam into the
intermediate grating, in which that beam is guided onto multiple
splitting regions of the intermediate grating in a direction along
the width of the intermediate grating. The intermediate grating is
arranged to split that beam at the splitting regions to provide
multiple substantially parallel versions of that beam. Those
multiple versions are coupled into the exit grating, in which the
multiple versions are guided onto multiple exit regions of the exit
grating. The exit regions lie in a direction along the width of the
exit grating. The exit grating is arranged to diffract the multiple
versions of that beam outwardly, substantially in parallel and in
an outward direction which substantially matches the unique inward
direction in which that beam was incoupled. The multiple input
beams thus cause multiple exit beams to exit the waveguide which
form substantially the same virtual version of the image.
FIG. 7a shows a perspective view of the display 15, imaging optics
17 and incoupling SRG 52. Different geometric points on the region
of the display 15 on which an image is displayed are referred to
herein as image points, which may be active (currently emitting
light) or inactive (not currently emitting light). In practice,
individual pixels can be approximated as image points.
The imaging optics 17 can typically be approximated as a principal
plane (thin lens approximation) or, in some cases, more accurately
as a pair of principal planes (thick lens approximation) the
location(s) of which are determined by the nature and arrangement
of its constituent lenses. In these approximations, any refraction
caused by the imaging optics 17 is approximated as occurring at the
principal plane(s). To avoid unnecessary complication, principles
of various embodiments will be described in relation to a thin lens
approximation of the imaging optics 17, and thus in relation to a
single principal plane labelled 31 in FIG. 7a, but it will be
apparent that more complex imaging optics that do not fit this
approximation still can be utilized to achieve the desired
effects.
The imaging optics 17 has an optical axis 30 and a front focal
point, and is positioned relative to the optical component 10 so
that the optical axis 30 intersects the incoupling SRG 52 at or
near the geometric centre of the incoupling SRG 52 with the front
focal point lying substantially at an image point X.sub.0 on the
display (that is, lying in the same plane as the front of the
display). Another arbitrary image point X on the display is shown,
and principles underlying various embodiments will now be described
in relation to X without loss of generality. In the following, the
terminology "for each X" or similar is used as a convenient
shorthand to mean "for each image point (including X)" or similar,
as will be apparent in context.
When active, image points--including the image point labelled X and
X.sub.0--act as individual illumination point sources from which
light propagates in a substantially isotropic manner through the
half-space forward of the display 15. Image points in areas of the
image perceived as lighter emit light of higher intensity relative
to areas of the image perceived as darker. Image points in areas
perceived as black emit no or only very low intensity light
(inactive image points). The intensity of the light emitted by a
particular image point may change as the image changes, for
instance when a video is displayed on the display 15.
Each active image point provides substantially uniform illumination
of a collimating area A of the imaging optics 17, which is
substantially circular and has a diameter D that depends on factors
such as the diameters of the constituent lenses (typically D is of
order 1-10 mm). This is illustrated for the image point X in FIG.
7a, which shows how any propagating light within a cone 32(X) from
X is incident on the collimating area A. The imaging optics
collimates any light 32(X) incident on the collimating area A to
form a collimated beam 34(X) of diameter D (input beam), which is
directed towards the incoupling grating 52 of the optical component
10. The beam 34(X) is thus incident on the incoupling grating 52. A
shielding component (not shown) may be arranged to prevent any
un-collimated light from outside of the cone 32(X) that is emitted
from X from reaching the optical component 10.
The beam 34(X) corresponding to the image point X is directed in an
inward propagation direction towards the incoupling SRG 52, which
can be described by a propagation vector {circumflex over
(k)}.sub.in (X) (herein, bold typeface is used to denote
3-dimensional vectors, with hats on such vectors indicating
denoting a unit vector). The inward propagation direction depends
on the location of X in the image and, moreover, is unique to X.
That unique propagation direction can be parameterized in terms of
an azimuthal angle .phi..sub.in(X) (which is the angle between the
x-axis and the projection of {circumflex over (k)}.sub.in (X) in
the xy-plane) and a polar angle .theta..sub.in(X)(which is the
angle between the z-axis and {circumflex over (k)}.sub.in (P) as
measured in the plane in which both the z-axis and {circumflex over
(k)}.sub.in (X) lie--note this is not the xz-plane in general). The
notation .phi..sub.in(X), .theta..sub.in(X) is adopted to denote
the aforementioned dependence on X; as indicated .phi..sub.in(X),
.theta..sub.in(X) are unique to that X. Note that, herein, both
such unit vectors and such polar/azimuthal angle pairs
parameterizing such vectors are sometimes referred herein to as
"directions" (as the latter represent complete parameterizations
thereof), and that sometimes azimuthal angles are referred to in
isolation as xy-directions for the same reason. Note further that
"inward" is used herein to refer to propagation that is towards the
waveguide (having a positive z-component when propagation is
towards the rear of the waveguide as perceived by the viewer and a
negative z-component when propagation is towards the front of the
waveguide).
The imaging optics has a principle point P, which is the point at
which the optical axis 30 intersects the principal plane 31 and
which typically lies at or near the centre of the collimation area
A. The inward direction {circumflex over (k)}.sub.in (X) and the
optical axis 30 have an angular separation .beta.(X) equal to the
angle subtended by X and X.sub.0 from P.
.beta.(X)=.theta..sub.in(X) if the optical axis is parallel to the
z-axis (which is not necessarily the case).
As will be apparent, the above applies for each active image point
and the imaging optics is thus arranged to substantially collimate
the image which is currently on the display 15 into multiple input
beams, each corresponding to and propagating in a unique direction
determined by the location of a respective active image point
(active pixel in practice). That is, the imaging optics 17
effectively converts each active point source X into a collimated
beam in a unique inward direction {circumflex over (k)}.sub.in (X).
As will be apparent, this can be equivalently stated as the various
input beams for all the active image points forming a virtual image
at infinity that corresponds to the real image that is currently on
the display 17. A virtual image of this nature is sometimes
referred to herein as a virtual version of the image (or
similar).
The input beam corresponding to the image point X.sub.0 (not shown)
would propagate parallel to the optical axis 30, towards or near
the geometric centre of the incoupling SRG 52.
As mentioned, in practice, individual pixels of the display 15 can
be approximated as single image points. This is illustrated in FIG.
7B which is a schematic plan view showing the principal plane 31
and two adjacent pixels Xa, Xb of the display 15, whose centres
subtend an angle .DELTA..beta. from the principal point P. Light
emitted the pixels Xa, Xb when active is effectively converted into
collimated beams 34(Xa), 34(Xb) having an angular separation equal
to .DELTA..beta.. As will be apparent, the scale of the pixels Xa,
Xb has been greatly enlarged for the purposes of illustration.
The beams are highly collimated, having an angular range no greater
than the angle subtended by an individual pixel from P
(.about..DELTA..beta.) e.g. typically having an angular range no
more than about 1/2 milliradian. As will become apparent in view of
the following, this increases the image quality of the final image
as perceived by the wearer.
FIGS. 7C and 7D show schematic plan (xz) and frontal (yz) views of
part of the optical component respectively. As indicated in these
figures, the incoupling grating 52 causes diffraction of the beam
34(X) thereby causing a first (.+-.1) order mode beam to propagate
within the optical component 10 in a new direction {circumflex over
(k)}(X) that is generally towards the fold SRG 54 (i.e. that has a
positive x-component). The new direction {circumflex over (k)}(X)
can be parameterized by azimuthal and polar angles .phi.(X)--where
|.phi.(X)|.ltoreq.|.phi..sub.in(X)| and .theta.(X)--where
|.theta.(X)|>|.theta..sub.in(X)|--which are also determined by
the location of and unique to the image point X. The grating 52 is
configured so that the first order mode is the only significant
diffraction mode, with the intensity of this new beam thus
substantially matching that of the input beam. As mentioned above,
a slanted grating can be used to achieve this desired effect (the
beam as directed away from the incoupling SRG 52 would correspond,
for instance, to beam T1 as shown in FIG. 4B or 4C). In this
manner, the beam 34(X) is coupled into the incoupling zone 12 of
the optical component 10 in the new direction {circumflex over
(k)}(X).
The optical component has a refractive index n and is configured
such that the polar angle .theta.(X) satisfies total internal
reflection criteria given by: sin 0(X)>1/n for each X. (1): As
will be apparent, each beam input from the imaging optics 17 thus
propagates through the optical component 10 by way of total
internal reflection (TIR) in a generally horizontal (+x) direction
(offset from the x-axis by .phi.(X)<.phi..sub.in(X)). In this
manner, the beam 34(X) is coupled from the incoupling zone 12 into
the fold zone 14, in which it propagates along the width of the
fold zone 14.
FIG. 7E shows 10 a frontal (xy) view of the whole of the optical
component 10, from a viewpoint similar to that of the wearer. As
explained in more detail below, a combination of diffractive beam
splitting and total internal reflection within the optical
component 10 results in multiple versions of each input beam 34(X)
being outwardly diffracted from the exit SRG along both the length
and the width of the exit zone 16 as output beams 38(X) in
respective outward directions (that is, away from the optical
component 10) that substantially match the respective inward
direction {circumflex over (k)}.sub.in (X) of the corresponding
input beam 34(X).
In FIG. 7E, beams external to the optical component 10 are
represented using shading and dotted lines are used to represent
beams within the optical component 10. Perspective is used to
indicate propagation in the z-direction, with widening (resp.
narrowing) of the beams in FIG. 7E representing propagation in the
positive (resp. negative) z direction; that is towards (resp. away
from) the wearer. Thus, diverging dotted lines represent beams
within the optical component 10 propagating towards the front wall
of the optical component 10; the widest parts represent those beams
striking the front wall of the optical component 10, from which
they are totally internally reflected back towards the rear wall
(on which the various SRGs are formed), which is represented by the
dotted lines converging from the widest points to the narrowest
points at which they are incident on the rear wall. Regions where
the various beams are incident on the fold and exit SRGs are
labelled S and E and termed splitting and exit regions respectively
for reasons that will become apparent.
As illustrated, the input beam 34(X) is coupled into the waveguide
by way of the aforementioned diffraction by the incoupling SRG 52,
and propagates along the width of the incoupling zone 12 by way of
TIR in the direction .phi.(X), .+-..theta.(X) (the sign but not the
magnitude of the polar angle changing whenever the beam is
reflected). As will be apparent, this results in the beam 34(X)
eventually striking the fold SRG at the left-most splitting region
S.
When the beam 34(X) is incident at a splitting region S, that
incident beam 34(X) is effectively split in two by way of
diffraction to create a new version of that beam 42(X)
(specifically a -1 reflection mode beam) which directed in a
specific and generally downwards (-y) direction .phi.'(X),
.+-..theta.'(X) towards the exit zone 16 due to the fold SRG 54
having a particular configuration which will be described in due
course, in addition to a zero order reflection mode beam (specular
reflection beam), which continues to propagate along the width of
the beam in the same direction .phi.(X), .+-..theta.(X) just as the
beam 34(X) would in the absence of the fold SRG (albeit at a
reduced intensity). Thus, the beam 34(X) effectively continues
propagates along substantially the whole width of the fold zone 14,
striking the fold SRG at various splitting regions S, with another
new version of the beam (in the same specific downward direction
.phi.'(X), .+-..theta.'(X)) created at each splitting region S. As
shown in FIG. 7E, this results in multiple versions of the beam
34(X) being coupled into the exit zone 16, which are horizontally
separated so as to collectively span substantially the width of the
exit zone 16.
As also shown in FIG. 7E, a new version 42(X) of the beam as
created at a splitting region S may itself strike the fold SRG
during its downward propagation. This will result in a zero order
mode being created which continues to propagate generally downwards
in the direction .phi.'(X), .+-..theta.'(X) and which can be viewed
as continued propagation of that beam, but may also result in a
non-zero order mode beam 40(X) (further new version) being created
by way of diffraction. However, any such beam 40(X) created by way
of such double diffraction at the same SRG will propagate in
substantially the same direction .phi.(X), .+-..theta.(X) along the
width of the fold zone 14 as the original beam 34(X) as coupled
into the optical component 10 (see below). Thus, notwithstanding
the possibility of multiple diffractions by the fold SRG,
propagation of the various versions of the beam 34(X)
(corresponding to image point X) within the optical component 10 is
effectively limited to two xy-directions: the generally horizontal
direction (.phi.(X), .+-..theta.(X)), and the specific and
generally downward direction (.phi.'(X), .+-..theta.'(X)) that will
be discussed shortly.
Propagation within the fold zone 14 is thus highly regular, with
all beam versions corresponding to a particular image point X
substantially constrained to a lattice like structure in the manner
illustrated.
The exit zone 16 is located below the fold zone 14 and thus the
downward-propagating versions of the beam 42(X) are coupled into
the exit zone 16, in which they are guided onto the various exit
regions E of the output SRG. The exit SRG 56 is configured so as,
when a version of the beam strikes the output SRG, that beam is
diffracted to create a first order mode beam directed outwardly
from the exit SRG 56 in an outward direction that substantially
matches the unique inward direction in which the original beam
34(X) corresponding to image point X was inputted. Because there
are multiple versions of the beam propagating downwards that are
substantially span the width of the exit zone 16, multiple output
beams are generated across the width of the exit zone 16 (as shown
in FIG. 7E) to provide effective horizontal beam expansion.
Moreover, the exit SRG 56 is configured so that, in addition to the
outwardly diffracted beams 38(X) being created at the various exit
regions E from an incident beam, a zero order diffraction mode beam
continuous to propagate downwards in the same specific direction as
that incident beam. This, in turn, strikes the exit SRG at a lower
exit zone 16s in the manner illustrated in FIG. 7E, resulting in
both continuing zero-order and outward first order beams. Thus,
multiple output beams 38(X) are also generated across substantially
the width of the exit zone 16 to provide effective vertical beam
expansion.
The output beams 38(X) are directed outwardly in outward directions
that substantially match the unique input direction in which the
original beam 34(X) is inputted. In this context, substantially
matching means that the outward direction is related to the input
direction in a manner that enables the wearer's eye to focus any
combination of the output beams 38(X) to a single point on the
retina, thus reconstructing the image point X (see below).
For a flat optical component (that is, whose front and rear
surfaces lie substantially parallel to the xy-plane in their
entirety), the output beams are substantially parallel to one
another (to at least within the angle .DELTA..beta. subtended by
two adjacent display pixels) and propagate outwardly in an output
propagation direction {circumflex over (k)}.sub.out(X) that is
parallel to the unique inward direction {circumflex over
(k)}.sub.in (X) in which the corresponding input beam 34(X) was
directed to the incoupling SRG 52. That is, directing the beam
34(X) corresponding to the image point X to the incoupling SRG 52
in the inward direction {circumflex over (k)}.sub.in (X) causes
corresponding output beams 38(X) to be diffracted outwardly and in
parallel from the exit zone 16, each in an outward propagation
direction {circumflex over (k)}.sub.out(X)={circumflex over
(k)}.sub.in (X) due to the configuration of the various SRGs (see
below).
As will now be described with reference to FIG. 7F, this enables a
viewer's eye to reconstruct the image when looking at the exit zone
16. FIG. 7F shows a plan (xz) view of the optical component 10. The
input beam 34(X) is in coupled to the optical component 10
resulting in multiple parallel output beams 38(X) being created at
the various exit regions E in the manner discussed above. This can
be equivalently expressed at the various output beams corresponding
to all the image points forming the same virtual image (at
infinity) as the corresponding input beams.
Because the beams 38(X) corresponding to the image point X are all
substantially parallel, any light of one or more of the beam(s)
38(X) which is received by the eye 37 is focussed as if the eye 37
were perceiving an image at infinity (i.e. a distant image). The
eye 37 thus focuses such received light onto a single retina point,
just as if the light were being received from the imaging optics 17
directly, thus reconstructing the image point X (e.g. pixel) on the
retina. As will be apparent, the same is true of each active image
point (e.g. pixel) so that the eye 37 reconstructs the whole image
that is currently on the display 15.
However, in contrast to receiving the image directly from the
optics 17--from which only a respective single beam 34(X) of
diameter D is emitted for each X--the output beams 39(X) are
emitted over a significantly wider area i.e. substantially that of
the exit zone 16, which is substantially larger than the area of
the inputted beam (.about.D.sup.2). It does not matter which
(parts) of the beam(s) 38(X) the eye receives as all are focused to
the same retina point--e.g., were the eye 37 to be moved
horizontally (.+-.x) in FIG. 7F, it is apparent that the image will
still be perceived. Thus, no adaptation of the display system is
required for, say, viewers with different pupillary distances
beyond making the exit zone 16 wide enough to anticipate a
reasonable range of pupillary distances: whilst viewers whose eyes
are closer together will generally receive light from the side of
the exit zone 16 nearer the incoupling zone 12 as compared with
viewers whose eyes are further apart, both will nonetheless
perceive the same image. Moreover, as the eye 27 rotates, different
parts of the image are brought towards the centre of the viewer's
field of vision (as the angle of the beams relative to the optical
axis of the eye changes) with the image still remaining visible,
thereby allowing the viewer to focus their attention on different
parts of the image as desired.
The same relative angular separation .DELTA..beta. exhibited the
input beams corresponding any two adjacent pixels Xa, Xb is also
exhibited by the corresponding sets of output beams 38(Xa),
38(Xb)--thus adjacent pixels are focused to adjacent retina points
by the eye 37. All the various versions of the beam remain highly
collimated as they propagate through the optical component 10,
preventing significant overlap of pixel images as focused on the
retina, thereby preserving image sharpness.
It should be noted that FIGS. 7A-7G are not to scale and that in
particular beams diameters are, for the sake of clarity, generally
reduced relative to components such as the display 15 than would
typically be expected in practice.
The configuration of the incoupling SRG 52 will now be described
with reference to FIGS. 8A and 8B, which show schematic plan and
frontal views of part of the fold grating 52. Note, in FIGS. 8A and
8B, beams are represented by arrows (that is, their area is not
represented) for the sake of clarity.
FIG. 8A shows two image points XL, XR located at the far left and
far right of the display 15 respectively, from which light is
collimated by the optics 17 to generate respective input beams
34(XL), 34(XR) in inward directions (.theta..sub.in(XL),
.phi..sub.in(XL)), (.theta..sub.in(XR), .phi..sub.in (XR)). These
beams are coupled into the optical component 10 by the incoupling
SRG 52 as shown--the incoupled beams shown created at the
incoupling SRG 52 are first order (+1) mode beams created by way of
diffraction of the beams incident on the SRG 52. The beams 34(XL),
34(XR) as coupled into the waveguide propagate in directions
defined by the polar angles .theta.(XL), .theta.(XR).
FIG. 8B shows two image points XR1 and XR2 at the far top-right and
far bottom-right of the display 15. Note in this figure
dashed-dotted lines denote aspects which are behind the optical
component 10 (-z). Corresponding beams 34(XL), 34(XR) in directions
within the optical component 10 with polar angles .phi.(XL),
.phi.(XR).
Such angles .theta.(X), .phi.(X) are given by the (transmissive)
grating equations: n sin .theta.(X)sin .phi.(X)=sin
.theta..sub.in(X)sin .phi..sub.in(X) (2)
.times..times..times..times..theta..function..times..times..times..PHI..f-
unction..times..times..theta..function..times..times..times..PHI..function-
..lamda. ##EQU00001## with the SRG 52 having a grating period
d.sub.1, the beam light having a wavelength .lamda., and n the
refractive index of the optical component.
It is straightforward to show from (2), (3) that
.theta.(XL)=.theta..sub.max and .theta.(XR)=.theta..sub.min i.e.
that any beam as coupled into the component 10 propagates with an
initial polar angle in the range [.theta.(XR), .theta.(XL)]; and
that .phi.(XR2)=.phi..sub.max and .phi.(XR1)=.phi..sub.min
(.apprxeq.-.phi..sub.max in this example) i.e. that any beam as
coupled into the component initially propagates with an azimuthal
angle in the range [.phi.(XR1), .phi.(XR2)](.apprxeq.[-.phi.(XR2),
.phi.(XR2)]).
The configuration of the fold SRG 54 will now be described with
references to FIGS. 9A-9B. Note, in FIGS. 9A and 9B, beams are
again represented by arrows, without any representation of their
areas, for the sake of clarity. In these figures, dotted lines
denote orientations perpendicular to the fold SRG grating lines,
dashed lines orientations perpendicular to the incoupling SRG
grating lines, and dash-dotted lines orientations perpendicular to
the exit SRG grating lines.
FIG. 9A shows a perspective view of the beam 34(X) as coupled into
the fold zone 14 of the optical component 10, having been reflected
from the front wall of the optical component 10 and thus travelling
in the direction (.phi.(X), -.theta.(X)) towards the fold SRG 54. A
dotted line (which lies perpendicular to the fold SRG grating
lines) is shown to represent the orientation of the fold SRG.
The fold SRG 54 and incoupling SRG 52 have a relative orientation
angle A (which is the angle between their respective grating
lines). The beam thus makes an angle A+.phi.(X) (see FIG. 9B) with
the fold SRG grating lines as measured in the xy-plane. The beam 34
is incident on the fold SRG 54, which diffracts the beam 34 into
different components. A zero order reflection mode (specular
reflection) beam is created which continues to propagate in the
direction (.phi.(X), +.theta.(X)) just as the beam 34(X) would due
to reflection in the absence of the fold SRG 54 (albeit at a
reduced intensity). This specular reflection beam can be viewed as
effectively a continuation of the beam 34(X) and for this reason is
also labelled 34(X). A first order (-1) reflection mode beam 42(X)
is also created which can be effectively considered a new version
of the beam.
As indicated, the new version of the beam 42(X) propagates in a
specific direction (.phi.'(X), .theta.'(X)) which is given by the
known (reflective) grating equations: n sin
.theta.'(X)sin(A+.phi.'(X))=n sin .theta.(X)sin(A+.phi.(X)) (4)
.times..times..times..times..theta.'.function..times..times..times..PHI.'-
.function..times..times..times..times..theta..function..times..times..time-
s..PHI..function..lamda. ##EQU00002## where the fold SRG has a
grating period d.sub.2, the beam light has a wavelength .lamda. and
n is the refractive index of the optical component 10.
As shown in FIG. 9B, which shows a schematic frontal view of the
optical component 10, the beam 34(X) is coupled into the incoupling
zone 12 with azimuthal angle .phi.(X) and thus makes an xy-angle
.phi.(X)+A the fold SRG 54.
A first new version 42a(X) (-1 mode) of the beam 34(X) is created
when it is first diffracted by the fold SRG 54 and a second new
version 42b(X) (-1 mode) when it is next diffracted by the fold SRG
54 (and so on), which both propagate in xy-direction .phi.'(X). In
this manner, the beam 34(X) is effectively split into multiple
versions, which are horizontally separated (across the width of the
fold zone 14). These are directed down towards the exit zone 16 and
thus coupled into the exit zone 16 (across substantially the width
of the exit zone 16 due to the horizontal separation). As can be
see, the multiple versions are thus incident on the various exit
regions (labelled E) of the exit SRG 56, which lie along the width
of the exit zone 16.
These new, downward (-y)-propagating versions may themselves meet
the fold SRG once again, as illustrated. However, it can be shown
from (4), (5) that any first order reflection mode beam (e.g.
40a(X), +1 mode) created by diffraction at an SRG of an incident
beam (e.g. 42a(X), -1 mode) which is itself a first order
reflection mode beam created by an earlier diffraction of an
original beam (e.g. 34(X)) at the same SRG will revert to the
direction of the original beam (e.g. .phi.(X), .+-..theta.(X),
which is the direction of propagation of 40a(X)). Thus, propagation
within the fold zone 14 is restricted to a diamond-like lattice, as
can be seen from the geometry of FIG. 9B. The beam labelled 42ab(X)
is a superposition of a specular reflection beam created when
42b(X) meets the fold SRG 54 and a -1 mode beam created when 40a(X)
meets the fold SRG at substantially the same location; the beam
labelled 42ab(X) is a superposition of a specular reflection beam
created when 40a(X) meets the fold SRG 54 and a +1 mode beam
created when 42b(X) meets the fold SRG at substantially the same
location (and so on).
The exit SRG and incoupling SRG 52, 56 are oriented with a relative
orientation angle A' (which is the angle between their respective
grating lines). At each of the exit regions, the version meeting
that region is diffracted so that, in addition to a zero order
reflection mode beam propagating downwards in the direction
.phi.'(X), .+-..theta.'(X), a first order (+1) transmission mode
beam 38(X) which propagates away from the optical component 10 in
an outward direction .phi..sub.out(X), .theta..sub.out (X) given
by: sin .theta..sub.out(X)sin(A'+.phi..sub.out(X))=n sin
.theta.'(X)sin(A'+.phi.'(X)) (6)
.times..times..theta..function..times..function.'.PHI..function..times..t-
imes..times..times..theta.'.function..times..function.'.PHI.'.function..la-
mda. ##EQU00003##
The output direction .theta..sub.out(X), .phi..sub.out(X) is that
of the output beams outside of the waveguide (propagating in air).
For a flat waveguide, equations (6), (7) hold both when the exit
grating is on the front of the waveguide--in which case the output
beams are first order transmission mode beams (as can be seen,
equations (6), (7) correspond to the known transmission grating
equations)--but also when the exit grating is on the rear of the
waveguide (as in FIG. 7F)--in which case the output beams
correspond to first order reflection mode beams which, upon initial
reflection from the rear exit grating propagate in a direction
.theta.'.sub.out(X), .phi.'.sub.out(X) within the optical component
10 given by: n sin .theta.'.sub.out(X)sin(A'+.phi.'.sub.out(X))=n
sin .theta.'(X)sin(A'+.phi.'(X)) (6')
.times..times..times..times..theta.'.function..times..times..times.'.PHI.-
'.function..times..times..times..times..theta.'.function..times..times..ti-
mes.'.PHI.'.function..lamda.' ##EQU00004## these beams are then
refracted at the front surface of the optical component, and thus
exit the optical component in a direction .theta..sub.in (X),
.phi..sub.in(X) given by Snell's law: sin .theta..sub.out(X)=n sin
.theta.'.sub.out(X) (8) .phi.'.sub.out(X)=.phi..sub.out(X) (9) As
will be apparent, the conditions of equations (6), (7) follow
straight forwardly from (6'), (7'), (8) and (9). Note that such
refraction at the front surface, whilst not readily visible in FIG.
7F, will nonetheless occur in the arrangement of FIG. 7F. It can be
shown from the equations (2-7) that, when d=d.sub.1=d.sub.3 (10)
(that is, when the periods of the incoupling and exit SRGs 52, 56
substantially match); d.sub.2=d/(2 cos A); (11) and A'=2A; (12)
then (.theta..sub.out (X), .phi..sub.out(X))=(.theta..sub.in (X),
.phi..sub.in (X)). Moreover, when the condition
.times..times..times.>.lamda. ##EQU00005## is met, no modes
besides the above-mentioned first order and zero order reflection
modes are created by diffraction at the fold SRG 54. That is, no
additional undesired beams are created in the fold zone when this
criteria is met. The condition (13) is met for a large range of A,
from about 0 to 70 degrees.
In other words, when these criteria are met, the exit SRG 56
effectively acts as an inverse to the incoupling SRG 52, reversing
the effect of the incoupling SRG diffraction for each version of
the beam with which it interacts, thereby outputting what is
effectively a two-dimensionally expanded version of that beam 34(X)
having an area substantially that of the exit SRG 56
(>>D.sup.2 and which, as noted, is independent of the imaging
optics 17) in the same direction as the original beam was inputted
to the component 10 so that the outwardly diffracted beams form
substantially the same virtual image as the inwardly inputted beams
but which is perceivable over a much larger area.
In the example of FIG. 9B, A.apprxeq.45.degree. i.e. so that the
fold SRG and exit SRGs 54, 56 are oriented at substantially 45 and
90 degrees to the incoupling SRG 52 respectively, with the grating
period of the fold region
##EQU00006## However, this is only an example and, in fact, the
overall efficiency of the display system is typically increased
when A.gtoreq.50.degree..
The above considers flat optical components, but a suitably curved
optical component (that is, having a radius of curvature extending
substantially along the z direction) can be configured to function
as an effective lens such that the output beams 30(X) are and are
no longer as highly collimated and are not parallel, but have
specific relative direction and angular separations such that each
traces back to a common point of convergence--this is illustrated
in FIG. 7G, in which the common point of convergence is labelled Q.
Moreover, when every image point is considered, the various points
of convergence for all the different active image points lie in
substantially the same plane, labelled 50, located a distance L
from the eye 37 so that the eye 37 can focus accordingly to
perceive the whole image as if it were the distance L away. This
can be equivalently stated as the various output beams forming
substantially the same virtual version of the current display image
as the corresponding input beams, but at the distance L from the
eye 37 rather than at infinity. Curved optical components may be
particularly suitable for short-sighted eyes unable to properly
focus distant images.
Note, in general the "width" of the fold and exit zones does not
have to be their horizontal extent--in general, the width of a fold
or exit zone 14, 16 is that zone's extent in the general direction
in which light is coupled into the fold zone 14 from the incoupling
zone 12 (which is horizontal in the above examples, but more
generally is a direction substantially perpendicular to the grating
lines of the incoupling zone 12).
As indicated, phase distortions caused by diffractive optical
elements imprinted on the surface of a waveguide--such as the SRGs
52, 54, 56--can degrade image quality in a display system of the
kind described above. In accordance with the present disclosure,
this can be mitigated by introducing suitable height offsets (i.e.
in a direction substantially normal to the surface on which they
are present) of the optical elements relative to one other and
relative to the blank surface of the waveguide.
FIG. 12A shows a side view of a part of a first waveguide 10c of
one embodiment, which is suitable for use in a display system of
the kind described above. The waveguide 10c has a first diffractive
optical element O1 (e.g. one of the incoupling, fold or exit SRGs
52, 54, 56) and a second diffractive optical element (e.g. another
of the incoupling, fold or exit SRG 52, 54, 56) imprinted on its
surface (for example on the rear of the waveguide 10c, from the
perspective of the viewer). The gratings O1, O2 are separated by a
blank surface region B, which--as in the waveguide of FIGS. 10A and
10B--causes light to change phase by .phi.0 upon reflection from
the blank surface region B. The blank region B could for instance
be the region W of FIG. 3B between the incoupling and fold SRGs 52,
54, or the unlabelled region between the fold and exit SRGs 54,
56.
The optical elements have the same structure (in particular, the
same depths h1, h2.noteq.h1) as those in FIGS. 10A and 10B, with
the first optical element O1 causing light to change phase by
.phi.1=.phi.0-.DELTA..phi.1 upon reflection therefrom and the
second optical element O2 causing light to change phase by
.phi.2=.phi.0-.DELTA..phi.2 upon reflection therefrom.
The depths h1, h2.noteq.h1 are, in contrast to the apodized
gratings of FIG. 11A, substantially constant over the entire area
of the gratings O1, O2 respectively, right up to the edges of the
gratings O1, O2. Alternatively the depths may vary as a function of
position (x,y) i.e. as functions h1(x,y), h2(x,y), but nevertheless
falls sharply to zero at the edges of the grating i.e. with a
significantly steeper gradient than the apodized edges of the
regions A1, A2 in FIG. 11A.
Moreover, in contrast to the waveguides 10a, 10b of FIGS. 10A, 10B
and 11A, the optical elements O1 and O2 gratings are at offset
height with respect to the blank TIR surface. Each height offset is
selected such that the additional optical path length introduced by
that offset substantially matches the phase difference between
reflection from the respective grating region and TIR. The
additional optical path length is the product of the refractive
index n of the waveguide 10c and the additional distance which
light traverses as a result of the offset.
The gratings O1, O2 are offset by distances .DELTA.h1 and .DELTA.h2
in the z-direction (i.e. in a direction substantially normal to the
surface on which they are imprinted) respectively. The expanded
view at the top of FIG. 12A shows this offset in detail: in
contrast to FIG. 10A, the tops of the filling regions of O1 and O2
are not level with the blank surface portion B, but are offset from
B by .DELTA.h1 and .DELTA.h2 respectively.
The offsets .DELTA.h1 and .DELTA.h2 substantially match
.DELTA..phi.1 and .DELTA..phi.2 respectively. That is, each offset
.DELTA.h1, .DELTA.h2 is such as to increase the length of the
optical path traversed by a ray R1, R2 reflected at the respective
grating O1, O2 relative to a ray R0 reflected at the blank surface
region by an amount that compensates for the differences in the
phase changes caused by reflection at O1, B, O2. For the grating O1
(resp. O2), the offset 4h1 (resp. .DELTA.h2) is such as to increase
the optical path length traversed by a ray R1 reflected at the
first grating O1 (resp. a ray R2 reflected at the second grating
O2) relative to that traversed by a ray R0 reflected at the blank
surface B by an amount over which the phase of the phase of the ray
R1 (resp. R2) changes by substantially .DELTA..phi.1
(resp..apprxeq..DELTA..phi.2). The optical path length traversed by
the ray R1 reflected from the first grating O1 is thus increased
relative to that traversed by the ray R2 reflected from the second
grating O2 by an amount over which the phase of the ray R1 changes
by substantially .DELTA..phi.1-.DELTA..phi.2. Phase matching does
not need to be completely accurate to achieve acceptable image
quality: phase changes from gratings and the TIR will be angle and
wavelength dependent which means that `fully` optimal performance
is obtained only for one case; for others is it less-optimal but
nonetheless acceptable in terms of final image quality. In practice
the system will be designed to meet conflicting requirements in
accordance with normal design practice.
A plane 90 is shown, which is perpendicular to the plane of the
waveguide 10c. As will be apparent, assuming the rays R1, R0, R2
are in phase with one another when they arrive at the plane 90
prior to reflection at O1, O2 and B respectively (at points P1, P0,
P2 respectively), when the offsets .DELTA.h1, .DELTA.h2
substantially match .DELTA..phi.1, .DELTA..phi.2 respectively in
the above described manner, the rays R1, R0, R2 will also be
substantially in phase with one another when they arrive at the
plane 90 again (at points Q1, Q0, Q2 respectively) after being
reflected. This will be true for any such plane lying below the
gratings O1, O2 and above the surface opposite the gratings (in
this case the front of the waveguide 10c).
The resulting phase distribution of reflected beams within the
waveguide 10c will thus be flat (as shown in FIG. 12B), without any
phase jumps that would cause unwanted diffractive beam
spreading.
The height offsets can be effected during manufacture, whereby a
substrate from which the waveguide 10c is made is processed so that
the gratings O1, O2 have the desired height offsets .DELTA.h1,
.DELTA.h2. The grating offset can be effected by an etching
process, for example, so that the blank area is offset from the
grating areas by the desired amount.
FIGS. 13A and 13B show side and plan views of a part of a second
waveguide 10d in another embodiment. In this example, first and
second optical elements O1 are substantially contiguous e.g.
separated by a blank region of no more than W.sub.max.apprxeq.100
.mu.m, and in some cases no more than 50 .mu.m. For example, the
optical elements O1, O2 could be the incoupling and fold SRGs 52,
54 of FIG. 3B, with W.ltoreq.Wmax. The first optical element 10d is
offset relative to the second optical component 10d by an amount
.DELTA.h' which substantially matches .DELTA..phi.1-.DELTA..phi.2
(which is also equal substantially equal to .DELTA.h1-.DELTA.h2)
i.e. the offset .DELTA.h' is such as to increase the optical path
length traversed by a ray R1 reflected at the first grating O1
relative to a ray R2 reflected at the second grating O2 by an
amount over which the phase of the phase of the ray R1 changes by
substantially .DELTA..phi.1-.DELTA..phi.2.
The expanded view at the top of FIG. 13 A shows a small blank
region b (e.g. .ltoreq.Wmax) separating the gratings O1, O2. When
the blank region b separating the gratings O1, O2 is sufficiently
small, the improvements in image quality can generally be realized
just by matching .DELTA.h' to .DELTA..phi.1-.DELTA..phi.2 without
having to worry about the offsets relative to the small blank
region b as the effects of b may be negligible.
A plane 90 is also shown in FIG. 13A, equivalent to that of 12A.
For any such plane 90, when .DELTA.h' is thus configured, rays R1,
R2 which arrive at the plane 90 (at points P1, P2) in phase with
one another prior to reflection at O1, O2 will also be
substantially in phase when they arrive at the plane 90 again (at
points Q1, Q2) following reflection.
FIG. 14 and FIG. 15 show simulated results for example grating
designs both with (FIG. 15) and without (FIG. 14) offset height.
FIG. 14 corresponds to the waveguide 10a of FIGS. 10A and 10B, and
FIG. 15 to the first waveguide 10c of FIG. 12A. The graphs labelled
a) show a simulated phase distributions for each waveguide; the
graphs labelled b) show the corresponding point spread functions
(PSF), and c) the corresponding modulation transfer functions
(MTF).
The PSF describes the response of an imaging system to a point
source or point object. In this case, the response is measured in
term of angle which represents the extent to which beam
de-collimation occurs within the waveguides i.e. beam spreading due
to diffraction. As will be apparent, a narrower PSF means less
de-collimation, and thus a sharper image.
The MTF is a measure of the ability of an optical system to
transfer various levels of detail from object to image. A
theoretical MTF of 1.0 (or 100%) represents perfect contrast
preservation (in practice, not achievable due to diffraction
limits), whereas values less than this mean that more and more
contrast is being lost--until an MTF of in practice around 0.1 (or
10%) when separate lines cannot be distinguished, peaks merge
together etc.
As can be seen from FIGS. 14 and 15, with the height offset the
waveguide has both a narrower PSF and good MTF over a larger range,
which is indicative of improved image quality.
It should be noted that light reflected from an optical element may
experience a zero phase change i.e. the optical element may cause
light to change phase upon reflection by an amount which is zero.
For the avoidance of doubt, it should be noted that, in the
following, when a structure is described as a causing light to
change phase upon reflection by an amount, that amount may or may
not be zero.
Note that the above arrangement of the light engine 13 is just an
example. For example, an alternative light engine based on
so-called scanning can provide a single beam, the orientation of
which is fast modulated whilst simultaneously modulating its
intensity and/or colour. As will be apparent, a virtual image can
be simulated in this manner that is equivalent to a virtual image
that would be created by collimating light of a (real) image on a
display with collimating optics.
Making an optical component which includes SRGs typically involves
the use of microfabrication techniques. Microfabrication refers to
the fabrication of desired structures of micrometer scales and
smaller. Microfabrication may involve etching of and/or deposition
on a substrate, to create the desired microstructure on the
substrate.
Wet etching involves using a liquid etchant to selectively dislodge
parts of a substrate e.g. parts of a film deposited on a surface of
a plate and/or parts of the surface of the plate itself. The
etchant reacts chemically with the substrate e.g. plate/film to
remove parts of the substrate e.g. plate/film that are exposed to
the etchant. The selective etching may be achieved by depositing a
suitable protective layer on the substrate/film that exposes only
parts of the substrate e.g. plate/film to the chemical effects of
etchant and protects the remaining parts from the chemical effects
of the etchant. The protective layer may be formed of a photoresist
or other protective mask layer.
Dry etching involves selectively exposing a substrate e.g.
plate/film (e.g. using a similar photoresist mask) to a bombardment
of energetic particles to dislodge parts of the substrate e.g.
plate/film that are exposed to the particles (sometimes referred to
as "sputtering"). An example is ion beam etching in which parts are
exposed to a beam of ions. Those exposed parts may be dislodged as
a result of the ions chemically reacting with those parts to
dislodge them (sometimes referred to as "chemical sputtering")
and/or physically dislodging those parts due to their kinetic
energy (sometimes referred to as "physical sputtering").
In contrast to etching, deposition--such as ion-beam deposition or
immersion-based deposition--involves applying material to rather
than removing material from a substrate e.g. plate/film. As used
herein, the term "patterning a substrate's surface" or similar
encompasses all such etching of/deposition on a plate or film, and
such etching of/deposition on a plate or film is said to impose
structure on the substrate's surface.
Conventional techniques for making an optical component involve,
for instance, first coating a to-be patterned region of a master
plate's surface (desired surface region) in a chromium layer or
other protective mask layer (e.g. another metallic layer). The
master plate and film constitute a substrate. The mask layer is
covered in a positive photoresist. Positive photoresist means
photoresist which becomes developable when exposed to light i.e.
photoresist which has a composition such that those parts which
have been exposed to light (and only those parts) are soluble in a
developing fluid used to develop the photoresist following
exposure. Light which forms a desired grating pattern (grating
structure)--created, for instance, using two-beam laser
interference to generate light which forms a grating structure in
the form of an interference pattern--is then projected onto the
photoresist so that only the photoresist at the locations of the
light bands is exposed. The photoresist is then developed to remove
the exposed parts, leaving selective parts of the mask layer
visible (i.e. revealing only selective parts) and the remaining
parts covered by the unexposed photoresist at the locations of the
dark fringes. The uncovered parts of the mask layer are then be
removed using conventional etching techniques e.g. an initial wet
etching or ion beam etching process which removes the uncovered
parts of the mask but not the parts covered by the photoresist, and
which does not substantially affect the plate itself. Etching of
the plate itself--such as further wet etching or further ion beam
etching--is then performed, to transfer the pattern from the etched
mask layer to the substrate itself.
FIG. 17A shows another optical component 10' which is similar in
some respects to the optical component 10 of FIGS. 3A and 3B, but
with some important differences that will now be discussed. As
illustrated, the other optical component 10' has SRGs 52'
(incoupling), 54' (fold), 56' (exit) similar to those of the
optical component 10, with large gaps (>>100 .mu.m) between
them, including between the incoupling and fold SRGs 52', 54'.
Because of this large spacing, in manufacturing the other optical
component 10', the laser interference exposure could be done, using
a positive photoresist technique along the lines of that outlined
above, simply by applying shadow masks of different shapes in front
of a master plate (substrate) during laser interference
exposure.
This is illustrated in FIG. 17B, which shows a master plate 70'
from the side during a two-beam laser interference exposure
process. The plate 70' is coated in a chromium layer 72', which is
itself coated in photoresist 74', which is positive photoresist.
The plate 70' and film 72' constitute a substrate. An interference
pattern is created by the interference of two laser beams 67i,
67ii. A shadow mask 69' is used to prevent the pattern from falling
outside of a desired portion (e.g. that spanned by incoupling SRG
52') of the substrate's surface so that the only photoresist which
is exposed is the parts covering the desired portion on which light
bands of the interference pattern fall (exposed photoresist is
shown in black and labelled 74'e in FIG. 17B). This can then be
repeated for any other portions to be patterned (e.g. for those
spanned by 54' and 56'). The positive photoresist can then be
developed to remove the exposed parts 74'e, and the substrate
patterned in the manner outlined above.
The shadow mask, however, causes distortion near the edges of the
DOE areas. The distortion is due to light scattering, non-perfect
contact of shadow mask and the finite thickness of the shadow mask
(which effectively blurs the pattern near its edge). Herein,
non-uniformity of a grating structure exhibited near its edges (of
the type caused by such shadowing during fabrication, or similar)
is referred to as "edge distortion". Edge distortion is indicated
by the label D in FIG. 17B.
When the photoresist is developed, the edge distortion becomes
embodied in the developed photoresist along with the grating
structure, and as a result is transferred to the surface of the
plate 70' when it comes to etching. As a result, the final optical
component 10' (which either comprises or is manufactured from the
patterned plate) also exhibits corresponding edge distortion as
indicated by the dotted lines labelled D around the edges of the
various DOE areas in FIG. 17A.
Moreover, as well as creating edge distortion, it is difficult to
position the shadow mask 69' accurately when exposing the substrate
in this manner, and therefore it would be difficult to reduce the
size of the gaps between the SRGs 52', 54' without risking overlap
between the SRGs 52', 54'.
Returning to FIG. 3B, in contrast to the other optical component
10' of FIG. 17A, the incoupling and fold zones 12, 14 of the
optical component 10 are substantially contiguous in that they are
separated by at most a narrow border zone 18 which has a width W as
measured along (that is, perpendicular to) a common border 19 that
divides the border zone 18. That is, the incoupling and fold zones
are separated by a small distance W in width along a common border
18. Moreover, the incoupling, fold and exit SRGs 52,54, 56 of the
optical component 10 are free from edge distortion of the kind
described above. It has been observed that this configuration
produces superior image quality to that of the other optical
component 10'.
In particular, it has been observed that, when the separation W of
the incoupling and fold regions 12, 14 along the common border 19
(the gap) is reduced to W.ltoreq.W.sub.max along the length of the
common border 19 (that is, provided the incoupling and fold zones
are separated by no more than W.sub.max in width along the length
of the common border 19)--where W.sub.max.apprxeq.50 .mu.m
(micrometers)--an improvement in image quality can be obtained. In
practice, the size of gap at which the improvement is observed may
have some dependence on the thickness of the waveguide. For
example, for a waveguide having a thickness (extent in the z
direction, as it is shown in the figures) of approximately 0.6 mm
or less, a dramatic improvement in image quality is observed when
W.sub.max is approximately 50 .mu.m or less. This particular case
is illustrated in FIG. 10, which shows curve of MTF (modular
transfer function) drop as function of gap width in one case
included for FIG. 18. The increase in MTF as the gap is reduced
from 50 .mu.m is immediately evident in FIG. 18. As is well known
to persons skilled in the art, the modular transfer function (MTF)
is a measure of the ability of an optical system to transfer
various levels of detail from object to image. An MTF of 1.0 (or
100%) represents perfect contrast preservation, whereas values less
than this mean that more and more contrast is being lost--until an
MTF of 0 (or 0%), where line pairs (a line pair is a sequence of
one black line and one white line) can no longer be distinguished
at all. For a thicker waveguide--e.g. of thickness approximately 1
mm, an improvement is still expected for a gap size of up to 100
.mu.m.
The common border 19 of FIG. 3B is arcuate (substantially
semi-circular in this example), with the incoupling and fold
regions 12, 14 having edges which are arcuate (in this case,
substantially semi-circular) along the common border 19. The edge
of incoupling region 12 is substantially circular overall.
The disclosure recognizes that conventional microfabrication
techniques are ill suited to making the optical component 10 of
FIG. 3B. In particular, existing techniques are ill-suited to
making optical components exhibiting the requisite incoupling-fold
zone separation W.ltoreq.W.sub.max and which are free of edge
distortion whilst still accurately maintaining the desired angular
orientation relationship between the various SRGs 52, 54, and 56
described above with reference to FIG. 9B.
A microfabrication process for making an optical component will now
be described with reference to FIG. 16. The process of FIG. 16 can
be used to.
As will become apparent in view of the following, the process of
FIG. 16 can be used to make optical components of the type shown in
FIG. 3B with the requisite small spacing between incoupling and
fold zones, which are free from edge distortion, and which moreover
exhibit the desired angular orientation to a high level of
accuracy.
That is, this disclosure provides a novel interference lithographic
method, which enables grating to be manufactured on the surface of
an optical component that are spaced apart from one another by 100
micrometers or less. This is not achievable typically achievable
with traditional interference lithographic methods.
FIG. 16 shows on the left-hand side a flow chart for the process
and on the right-hand side, for each step of the process, plan
and/or side views of an exemplary master plate 70 as appropriate to
illustrate the manner in which the plate 70 is manipulated at that
step. Each side view is a cross-section taken along the dash-dotted
line shown in the corresponding plan view.
An upper part of the plate's surface is coated with a chromium film
72. The plate 70 and film 72 constitute a substrate, a desired
surface region of which (specifically, the surface region defined
by the chromium layer 72 in this example), in performing the
process, is selectively etched to create incoupling and fold SRGs
52, 54. The incoupling SRG 52 is fabricated on a first portion 62
of the desired surface region (incoupling portion), and the fold
SRG 54 on a second distinct (i.e. non-overlapping) and
substantially contiguous portion 64 of the desired surface region
(fold portion) having the reduced separation W.ltoreq.W.sub.max
along the (intended) common border 19. For the optical component 10
shown in FIGS. 3A and 3B, the desired region corresponds to the
rear of the component's surface from the perspective of the
wearer.
The final etched substrate constitutes an optical component which
may be incorporated in a display system (e.g. the display system 2
of FIG. 1), or which may be for use as a production master for
manufacturing further optical components e.g. a mould for moulding
such components from polymer (or indeed which may be used for
making such moulds), in which case the SRGs 52, 54 as fabricated on
the substrate's surface are transferred to (the rear of) those
components by the manufacturing e.g. moulding process.
At step S4 of FIG. 16, the chromium layer 72 is coated in a
negative photoresist film 74--that is, photoresist which becomes
undevelopable when exposed to light i.e. photoresist which has a
composition such that those parts which have been exposed to light
(and only those parts) become substantially insoluble in a
developing fluid used to develop the photoresist once exposed so
that the exposed parts (and only the parts) remain
post-development. This includes coating the incoupling portion 62
which is ultimately intended to be patterned with the incoupling
SRG 52, as well as the fold portion 64 ultimately intended to be
patterned with the fold SRG 54.
At step S6, an area substantially larger than and encompassing the
incoupling portion 62 is exposed (shown in this example as a
rectangle containing the desired circular area 62) to light which
forms the desired incoupling grating structure (i.e. that of SRG
52). By directing two laser beams 67i, 67ii to coincide in an
interference arrangement, an interference pattern which forms the
desired incoupling grating structure, having a grating period d
when incident on the photoresist 74, is created. The interference
pattern comprises alternating light and dark bands, whereby only
the parts of the photoresist on which the light bands fall are
exposed (exposed photoresist is shown in black and labelled 70e in
FIG. 16); however, in contrast to positive photoresist, it is these
exposed parts 70e which become undevelopable whereas the
non-exposed parts in the locations of the dark bands remain
developable.
A shadow mask 69 is used to restrict the interference pattern to
the larger area. The larger area is large enough not only to
encompass the incoupling surface portion 62 but also such that all
the edge distortion D created by the shadow mask lies outside of
the incoupling portion 62 (in general, it is sufficient for the
wider area to be such there is substantially no edge distortion in
the vicinity of the intended common border 19, even if there is
some edge distortion present elsewhere around the edge of the
incoupling portion 62).
A dummy grating portion 63 is also exposed to the same (or a
similar) interference pattern at the same time for reasons that
will be discussed in due course.
The exposed portions 62, 63 can be practically of any shape or size
but the excess exposure resulting from possible other exposures
must not reach any "active part" of the desired exposure portions
(i.e. in the illustration aside S6, other exposures must not
overlap the circular incoupling portion 62).
As an alternative to using masks, the interference pattern could be
projected over the whole of the desired surface region so that no
shadowing effects are present on the desired surface region at
all.
During the exposure step S6, the plate 70 is supported by a
mechanical clamping or other fixing method in an laser interference
exposure setup (exposure system) not shown in FIG. 16 to hold it
steady relative to the exposure system (in particular, relative to
the beams 67i, 67ii) whilst the exposure takes place. After step
S6, the master plate 70 is unloaded from the laser interference
exposure setup.
At step S8, the unloaded plate 70 is exposed to light 65 of
substantially uniform intensity, but with photo mask 80 in place to
expose photoresist and thus avoid photoresist development from
areas outside the incoupling and dummy grating areas 62, 63. That
is, photo mask 80 on the incoupling portion 62 and the dummy region
63 are used to prevent exposure of the portions 62, 63 to the
uniform light 65. Thus, uniform light 65 is projected over the
entirety of the desired surface region but for the incoupling and
dummy portions (as these are covered by the photo mask 80) so that
all of the photoresist other than that covering the incoupling and
dummy portions 62, 63 becomes undevelopable throughout. It is thus
the photo mask which define the portions 62, 63 (i.e. the portions
62, 63 have the same size and shape as the corresponding photomask
80 used to protect those portions), and not the shadow masks used
in S6. A mask aligner is used to position the photo mask 80
accurately on correct position on the substrate. The mask aligner
has components (e.g. ultraviolet-lamp, optics etc.) for generating
uniform light for exposure and the mechanics for positioning the
photomask 80 to the correct position.
As will be apparent, the only photoresist to retain any record of
the grating structure(s) as projected at S6 is that which covers
the incoupling and dummy portions--outside of those portions, all
record of the grating structure(s) is intentionally destroyed. The
entirely exposed photoresist outside of the incoupling and dummy
portions 62, 63 includes all the parts of the photoresist that were
subjected to the edge distortion D, thus completely removing any
record of the edge distortion from the photoresist. Due to the
nature of the process, there is virtually no distortion to the
grating pattern.
At step S10, the photoresist is developed to embody the incoupling
SRG grating structure by removing only those parts of that
photoresist that have not been exposed to light using a developing
fluid. All the exposed, undevelopable photoresist 74e is left
substantially unchanged by the development of step S10. As
illustrated in the figures to the right of S10 in FIG. 16,
substantially no photoresist outside of the portions 62, 63 is
removed in step S10; the only removed photoresist is lines of
unexposed photoresist in the incoupling and dummy portions 62, 63
corresponding to the locations of the dark bands of the
interference pattern as projected on the photoresist at S6.
At step S11, a chromium etching procedure is performed to etch the
chromium layer 72 (but not the plate 70 itself) with the incoupling
SRG pattern, such as dry etching of the chrome hard mask 72. In
etching step S11, the photoresist serves as an etching mask to
restrict etching of the chromium layer 72 to the incoupling and
dummy grating surface portions only, whereby structure is imposed
from the photoresist to the incoupling and dummy portions 62, 63.
However, the exposed, undeveloped photoresist 74e outside of the
portions 62, 63 inhibits etching outside of those portions 62, 63
so that no structure is imposed on the chromium 72 outside of those
portions 9 (i.e. outside of those portions, the chromium is
substantially unchanged).
Once the chromium 72 has been etched thus, the exposed photoresist
74e is removed (S12) and the chromium 72 recoated with fresh,
unexposed negative photoresist 74 (S13).
As indicated above, the relative orientation angle between
incoupling and fold SRGs is intended to be A as defined in equation
(11) above and shown in FIG. 9B (with the incoupling and exit SRGs
having a relative orientation angle 2A, as per equation (12)). This
can be achieved by re-loading the plate 70 in the same exposure
system (previously used at S6) supported again by the same
mechanical clamps or other fixing method, and rotating the plate 70
by an amount that matches A relative to the exposure system so that
any subsequently projected pattern is oriented to the original
incoupling SRG pattern by A (S14). By using a suitable drive
mechanism, it is possible to achieve highly accurate rotation of
the plate 70.
However, due to inaccuracy of mechanical stoppers, the position of
the plate 70 is not accurately the same as in step S6. This is
illustrated in the plan view aside step S14 of FIG. 16, in which an
angle .alpha. is shown to denote slight rotation relative to the
plate's initial orientation at the previous exposure step S6 caused
by unloading/reloading of the plate 70.
For this reason, prior to rotating the plate 70 at S14, the offset
.alpha. between the plate position in S6 and S14 is first measured.
The measurement is done using a moire pattern 81. The moire pattern
81 changes when the plate is rotated and this can be used to
measure the angle of the plate with better than 0.001 degrees
resolution.
To create the moire pattern 81, the dummy grating portion is
re-exposed to the same interference pattern it was exposed to at
step S6 (or at least an interference pattern having the same
angular orientation), as shown on the right-hand side of FIG. 16.
The moire pattern is evident notwithstanding the presence of the
photoresist atop the dummy grating. The moire pattern is created as
a result of the interaction between the interference pattern and
the dummy grating, and when the angular alignment is better than
e.g. 0.01 degrees, has a fringe spacing--typically of the order of
few mm--and thus clearly visible when the offset .alpha. is about 5
thousandths of a degree, and which increases as .alpha. is reduced
towards zero, becoming maximal (effectively infinite) upon .alpha.
reaching zero. The fringe spacing is determined by the offset
.alpha. and, conversely, can be used to measure .alpha..
This leaves the photoresist atop the dummy grating partially
exposed; as will become apparent, this is inconsequential. Notably,
the dummy grating portion 63 is sufficiently offset from the fold
grating portion 64 for the photoresist atop the fold grating
portion to remain unexposed in creating the moire pattern 81.
Once .alpha. has been measured, at step S16 the plate 70 is rotated
from that initial orientation by an amount=A-.alpha. (thereby
accounting for a in the rotation) so that the plate 70 now has an
orientation A relative to its initial position at S6 to a high
level of accuracy.
At step S18, an area substantially larger than and encompassing the
fold portion 64 is exposed (shown in this example as a rectangle
containing the desired area 64) again by directing two laser beams
67i, 67ii to coincide in an interference arrangement, leaving the
parts of the photoresist on which light bands fall undevelopable in
a manner equivalent to S6 (but without any additional dummy grating
area being exposed). In S18, the interference pattern has a period
d/(2 cos A) when incident on the photoresist. A shadow mask 69 is
again used to restrict the interference pattern to this area, which
is large enough not only to encompass the fold surface portion 64
but also such that all the edge distortion D created by the shadow
mask lies outside of the incoupling portion 62 (or at least clear
of the common border 16).
Some or all of the photoresist atop the incoupling grating will
likely be exposed at S18, which is inconsequential as it has no
effect on the incoupling pattern which has already been etched into
the underlying chromium 72.
All other areas except fold portion 64 are then exposed (S19) to
uniform light 65 with a suitable photo mask 80 in place to prevent
exposure of the fold portion 64 (and only that portion) in a manner
equivalent to step S8. This leaves all the photoresist covering the
incoupling portion 62 (and also that covering an exit portion
ultimately intended to be etched to form the exit grating 56)
exposed and therefore undevelopable. The photoresist is then
developed to remove only the unexposed parts (S20) in a manner
equivalent to step S10, the chromium one again etched to transfer
the fold SRG pattern from the photoresist to the chromium, and the
photoresist removed following etching (equivalent to S11-S12). The
incoupling portion is protected by the exposed and therefore
undeveloped photoresist 70e, thereby preserving the incoupling
grating pattern already etched into the chromium.
The use of photo mask 80 to define the incoupling and fold portions
enables the location of the DOE areas to be controlled far more
accurately then when simply using shadow masks to define those
areas (as in the positive photoresist technique outlined above). It
thus becomes possible to reduce the separation of those portions to
W.ltoreq.W.sub.max whilst still retaining separation of those
portions (i.e. without the etched patterns overlapping).
Although not shown explicitly in FIG. 16, it will be apparent that
the chromium covering the grating area ultimately intended for the
exit SRG 56 (vertically below the incoupling and fold SRGs 52, 54)
is unaffected by the etching of both 511 and S22 as in both of
those steps it is protected by undeveloped photoresist.
A similar process could be repeated to etch the desired fold
grating structure into the chromium, again using a moire pattern to
achieve a highly accurate angular orientation of 2 A between the
incoupling and exit grating structures. The exit grating in the
present configuration is relatively far away from the input
grating. Thus input grating and exit grating can be exposed to the
same photoresist layer with large enough shadow masks to avoid edge
distortions.
Once all three structures have been etched into the chromium, the
plate 70 itself is subject to an etching procedure (e.g. ion-beam
etching) in which the chromium now serves as an etching mask,
whereby the grating structures are transferred from the etched
chromium 72 to the plate 70 itself to form the desired incoupling,
exit and fold SRGs 52, 54, 56 on the plate itself with very good
angular accuracy, narrow gap W.ltoreq.W.sub.max between SRgs 52,
54, and good quality edges free form edge distortion.
Note that the dummy grating pattern is not etched onto the plate
itself as it is not desired on the final optical component.
Once the plate itself has been etched, the chromium is removed and
the plate 70, can e.g. be used in a display system of the kind
shown in FIG. 1, to mould further optical components, or indeed to
make such moulds.
It has been demonstrated that, using the process of FIG. 16,
substrates can be patterned, free from edge distortion, with the
actual relative orientation angle between the incoupling and fold
zones 14, 16 consistently being arccos(d.sub.1/(2d.sub.2)) (see
equations 11, 12 above) and/or one half of the relative orientation
angle between the incoupling and exit SRGs 12, 16 (see equation 13,
above) to within .+-.one thousandth of a degree (as measured from a
representative statistical population of substrates fabricated
using the present techniques). However two thousandths of a degree
may be still acceptable angular error in some practical contexts.
It should be noted that the subject matter is not limited to the
particular exit pupil expansion configuration or grating structures
but applies to other configurations as well. Moreover, whilst the
above is described with reference to diffraction gratings in the
form of SRGs, the subject matter is not limited to diffraction
gratings and encompasses any structures which cause different phase
changes.
In embodiments of the various aspects set out in the Summary
section, the structure of the first portion may constitute a first
diffraction grating. The structure of the second portion may also a
second diffraction grating.
The first grating may have a depth different from the second
grating.
The first grating may have a depth which is substantially constant
over the entire first portion up to the edge of the first grating.
The first grating may have a depth which is substantially constant
over the entire first portion up to the edge of the first grating,
and the second grating has a depth which is substantially constant
over the entire second portion up to the edge of the second
grating.
The structure of the first portion may constitute a first
diffraction grating and the structure of the second portion may be
substantially non-diffractive. The first grating may have a depth
which is substantially constant over the entire first portion up to
the edge of the first grating.
The first and second portions may be substantially contiguous.
The first and second portions may be separated by no more than 100
micrometers in width along a common border, and optionally no more
than 50 micrometers in width along the common border.
A third portion of the same surface may have a structure which
causes light to change phase upon reflection from the third portion
by a third amount different from the first amount, wherein the
first and third portions are adjacent the second portion so that
the second portion separates the first and third portions, and
wherein the third portion is offset from the second portion by a
distance which substantially matches the difference between the
second amount and the third amount.
The structure of the first portion may constitute a first
diffraction grating, the structure of the third portion may
constitute a second diffraction grating, and the structure of the
second portion may be substantially non-diffractive.
The structure of the first portion may constitute an incoupling
grating via which said light is coupled into the waveguide from the
display of the display system. The structure of the second portion
may constitute an exit grating via which said light exits the
waveguide onto the eye of the user. The structure of the second
portion may constitute an intermediate grating configured to
manipulate the spatial distribution of the light within the
waveguide.
Although the subject matter has been described in language specific
to structural features and/or methodological acts, it is to be
understood that the subject matter defined in the appended claims
is not necessarily limited to the specific features or acts
described above. Rather, the specific features and acts described
above are disclosed as example forms of implementing the
claims.
* * * * *
References